Battery spec discrepancies - is this a widespread practice?

General lounge
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Is someone well-versed in matters of electric charge versus current versus voltage?
I had a recent exchange with a battery manufacturer regarding what I thought was a fault in their product. Their response seems devious to me, but I could easily be wrong about that. Maybe to someone with knowledge it will be immediately apparent if the manufacturer is operating above board. ?

Here’s my question them:

I recently purchased your Aukey 12000mAh Portable Power Bank Charger External Battery Pack, but its performance is nowhere near your advertised specifications. The charge capacity of this device is only 8400 mAh, far from the 12000mAh you say it should be. (I measured this simply by using it to charge my iPad mini 3 which has a battery capacity of 6470 mAh. The fully-charged Aukey Power Bank was able to charge the ipad once to 100% and then a second time only to 30%.) Is this normal?

Here’s the manufacturer’s reply to my question:

Thank you for your email.
Please let us expalin it is how a 12000 mah battery actually ouput calculated

（3.7V * 12000mah）* 90% ÷ 5V=7992mah

Inside battery voltage of the battery is 3.7V
Output voltage of the aukey battery 5V

There must be power lose during discharge so the transfer rate is 90%

You can google about it for more details if you dont believe.
If there are any further questions,please feel free to tell us.
Kind regards,
Ann

Is this reasonable? It seems oddly misleading to spec a battery’s capacity at 12000mah when they know that it can only deliver a charge of 7992mah? Nowhere in their product description do they mention this discrepancy between internal voltage and output voltage, so their spec gives consumers no real information about the actual utility of their product. Is this normal practice for battery manufacturers?

Here’s a link to the product: http://www.amazon.ca/gp/product/B00LSKPLM8

Thanks! ( ͝° ͜ʖ͡°)

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[quote=“alexg, post:1, topic:3909”]
It seems oddly misleading to spec a battery’s capacity at 12000mah when they know that it can only deliver a charge of 7992mah?[/quote]

Yeah, unfortunately. Pretty much every Chinese manufacturer does this, especially on aliexpress.

It’s one of the reasons why I like FastTech.com, cause they will actually test the batteries and post the actual measured capacity.
image
https://www.fasttech.com/category/1420/batteries/-/p/2?f=-

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The Manufacture has explained it quite well.

mah is not a measure of energy

(mill watt hours) mwh is a measure of energy.

Batteries are sold with mah ratings for a specified voltage, This
gives the energy storage capacity in mwh.
This battery has energy storage of

12000mah *3600/1000 = 43200 C

43200C * 3.7V = 159840J

90% efficient DC-DC converter is quite good.

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I agree that the spec should say 5V battery source with 12000mah capacity @
3.7V

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thanks for the source

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But according to what Ann says, there’s actually no way to test the “internal” capacity of a battery unless you know its “internal” voltage. I.e. the battery may have a standard output voltage of 1.5v but if the internal voltage is 3v then its internal capacity might be double what it will actually deliver externally (e.g. 3000mah versus 1500mah). It sounds totally perverse.

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Thanks for your reply!
This leaves me wondering whether all manufacturers deceive in this and to this the same degree. If so, then at least the capacity specs will still provide a reasonable relative comparison between products. But if some deceive and some don’t, then the published product specs become totally useless for comparison purposes.